To record or reproduce information on an optical recording medium, light to be used is condensed by an objective lens and the very small light spot condensed has to be formed on the information recording surface of the optical recording medium. In recent years, with the progress of the realization of a large recording capacity in the optical recording medium, it is desired to form further smaller light spots. The light spot diameter is determined by a wavelength λ to be used and the numerical aperture NA of an objective lens, and it is proportional to wavelength/numerical aperture (λ/NA). Thus, an attempt has been proceeding in which the numerical aperture of the objective lens is increased, whereby the light spot diameter is reduced.
The coma aberration which is generated in the light having transmitted through the objective lens due to the tilt of the optical recording medium is increased in proportion to the third power of the numerical aperture. With the increase in the numerical aperture of the objective lens in this manner, the coma aberration generated due to the tilt of the optical recording medium is grown, thus causing a problem that good reproducibility cannot be obtained. Accordingly, to keep the optical recording medium and the optical axis of the light being condensed in the orthogonal state and to prevent the coma aberration from being generated, the amount of tilt in the optical recording medium needs to be firstly detected.
An example of the related art of detecting the amount of tilt in the optical recording medium is described as follows (see a patent literature 1). FIG. 40 is a partial cross section illustrating the simplified configuration of a traditional tilt sensing apparatus 1. FIG. 41 is a plan view illustrating the simplified configuration of a light sensing unit 2 provided in the traditional tilt sensing apparatus 1. In the traditional tilt sensing apparatus 1, a flat glass 4 is mounted on a lens holding member 5 that holds an objective lens 3. The light sensing unit 2 and a light shield 7 are fixed at positions of axial symmetry about the axis of the objective lens 3 on the objective lens 3 or flat glass 4. A light 8 emitted from a light source toward an optical recording medium 6 is partially shielded by the light shield 7, incident on the objective lens 3, and condensed onto the optical recording medium 6 by the objective lens 3. The light 8 emitted from the light source including a light shielded portion 9 is reflected at the position of axial symmetry by the optical recording medium 6. As shown in FIG. 41, the light sensing unit 2 is formed of four light receiving elements 10, 11, 12 and 13. When the optical recording medium 6 is not tilting, that is, it is orthogonal to an optical axis 14 coaxial to the axis of the objective lens 3, the position where the light shielded portion 9 of the light 8 emitted from the light source is reflected is matched with the position of the light sensing unit 2. Therefore, suppose detection signals detected by discrete light receiving elements 10 to 13 are expressed by detection signals a, b, c and d, respectively, and the difference between the detection signal a and the detection signal c, and the difference between the detection signal b and the detection signal d are zero.
FIG. 42 is a partial cross section simplifiedly illustrating the state that the optical recording medium 6 is tilting in the traditional tilt sensing apparatus 1. FIG. 43 is a plan view illustrating the position where the light shielded portion 9 reaches on the light sensing unit 2 in the state shown in FIG. 42.
As shown in FIG. 42, when the optical recording medium 6 is tilting in the counterclockwise direction on the paper of FIG. 42 in the tilt sensing apparatus 1, the light reflected by the optical recording medium 6 takes the optical path shifted in the right direction on the paper of FIG. 42. On this account, the light shielded portion 9 is reflected by the optical recording medium 6 and reaches at the position shifted in the right direction on the paper of FIG. 43 on the light sensing unit 2. Thus, the difference between the detection signal b and the detection signal d is positive. Inversely, when the optical recording medium 6 is tilting in the clockwise direction on the paper of the FIG. 42, the reflected light from the optical recording medium 6 takes the optical path shifted in the left direction on the paper of the FIG. 43 and the light shielded portion 9 reaches at the position shifted in the left direction on the light sensing unit 2. Thus, the difference between the detection signal b and the detection signal d is negative. Accordingly, from the results of operating the detection signals obtained by receiving light in the discrete light receiving elements 10 to 13, the amount of tilt in the optical recording medium 6 can be detected.
[Patent literature 1]
Japanese Unexamined Patent Publication JP-A 8-235624 (1996) (Pages 3–4, FIG. 2)
The traditional tilt sensing apparatus 1 of the related art has the following problems. In the tilt sensing apparatus 1, the light shield 7 and the light sensing unit 2 have to be disposed integrally in the objective lens 3 or lens holding member 5. Tracking control in recording or reproducing the optical recording medium 6 is performed by drive of an objective lens actuator mounted with the lens holding member 5. Accordingly, when the light sensing unit 2 is not disposed integrally in the objective lens 3 or lens holding member 5 as described above, the light shield 7 is moved with the tracking control, thereby causing the phenomenon that the light shielded portion 9 is shifted on the light sensing unit 2, despite the axis of the objective lens 3 being orthogonal to the optical recording medium 6. To prevent this phenomenon, the light shield 7 and the light sensing unit 2 have to be integrally combined with the objective lens 3 or lens holding member 5.
However, when the light sensing unit 2 and the light shield 7 are integrally mounted on the objective lens 3 or flat glass 4 held by the lens holding member 5, the light sensing unit 2 has to be routed through the objective lens actuator, thus causing a problem that the fabrication processes are complicated. In addition, a light sensing unit for detecting detection signals of information provided in the optical recording medium 6 and tracking error signals cannot be combined with the light sensing unit 2 into one, thus causing a problem that the parts count is increased and costs are raised. Furthermore, a plurality of light sensing units is disposed, thus causing a problem that the arrangement of optical systems is complicated.